According to the IAU’s General Assembly, which met in Prague this year, a celestial body can be called a planet only if a) it is in orbit around the Sun; b) it is round (in other words, is massive enough to be shaped into a ball by its own gravity); and c) it has cleared the neighbourhood around its own orbit.

It is c) that has proved awkward. The IAU meant that planets should orbit the Sun in isolation, rather than whizzing around with other bodies, such as asteroids. This clause did for Pluto, because it resides in the Kuiper Belt, a girdle of icy bodies that lies beyond Neptune.

But, Fienberg points out: “Our own world is threatened by . . . a host of other near-Earth asteroids whose paths around the Sun intersect ours. By strict application of the IAU’s new rules, this means Earth is no longer a planet either. Ditto for Mars, Jupiter and Neptune, all of which are accompanied in orbit by little asteroids. Ridiculous!”

Special plead your way out of that one, IAU. I confidently predict the appearance of a modified sub-clause which defines 'cleared' as 'not having any other bodies above a certain percentage of the size of the candidate planet' – if there isn’t one already. It’s not so much I’m convinced by this argument, but it just shows how easy it is to tie yourself in knots when you start preferring semantics to science. And this closing line is priceless:

Fienberg has come up with a fresh mnemonic, forged in pure bitterness, to describe the new Pluto-less planetary octet: Many Very Egotistical Malcontents Just Screwed Up Nomenclature.

3 comments:

Josh
said...

I've spoken to some of the people who were at the general assembly, and the intent of the resolution was that a planet gravitationally dominates its orbital zone around the Sun. I, too, found it surprising that both this definition and the earlier (and in my view, inferior) definitions were not air tight legalistic definitions, but rather fairly casual and semantically loose. It was apparently a conscious decision not to be overly rigorous about the definition in order to have something that is easy and straightforward, namely: orbit the Sun, be big enough to be round, be the dominant body in your orbit.

Jupiter shares its orbit with literally thousands of Trojan asteroids. The members of the IAU were well aware of this when they crafted the planet definition resolution.

Hmm, I'm not sure composition would work very well for "planet" itself, since I think community consensus favors keeping the terrestrial planets (composition: rock, metal) and giant planets (composition: H, He) both categorized as planets. Anyway, general usage over the course of many years will probably sort this out.